Extraordinary 13C enrichment of diether lipids at the Lost City Hydrothermal Field indicates a carbon-limited ecosystem

Active and inactive carbonate chimneys from the Lost City Hydrothermal Field contain up to 0.6% organic carbon with diverse lipid assemblages. The delta C-13 values of total organic carbon range from -21.5 parts per thousand vs. VPDB at an extinct carbonate chimney to -2.8 parts per thousand. at a 70 degrees C, actively venting carbonate chimney. Samples collected at locations with total organic carbon with delta C-13 > -15 parts per thousand also contained high abundances of isoprenoidal and nonisoprenoidal diether lipids. Samples with TOC more depleted in C-13 lacked or contained lower amounts of these diethers. Isoprenoidal diethers, including sn-2 hydroxyarchaeol, sn-3 hydroxyarchaeol, and putative dihydroxyarchaeol, are likely to derive from methanogenic archaea. These compounds have delta C-13 values ranging from -2.9 to +6.7 parts per thousand vs. VPDB. Nonisoprenoidal diethers and monoethers are presumably derived from bacteria, and have structures similar to those produced by sulfate-reducing bacteria in culture and at cold seeps. In samples that also contained abundant hydroxyarchaeols, these diethers have delta C-13 values between -11.8 and +3.6 parts per thousand. In samples without abundant hydroxyarchaeols, the nonisoprenoidal diethers were typically more depleted in C-13, with delta C-13 as low as -28.7 parts per thousand in chimneys and -45 parts per thousand in fissures. The diethers at Lost City are probably derived from hydrogen-consuming methanogens and bacteria. High hydrogen concentrations favor methanogenesis over methanotrophy and allow the concurrent growth of methanogens and sulfate-reducing bacteria. The unusual enrichment of C-13 in lipids can be attributed to nearly complete consumption of bioavailable carbon in vent fluids. Under carbon-limited conditions, the isotope effects that usually lead to C-13-depletion in organic material cannot be expressed. Consequently, metabolic products such as lipids and methane have delta C-13 values typical of abiotic carbon. (C) 2008 Elsevier Ltd. All rights reserved.